Monocrystalline rutile and a method for preparing the same



United States Patent l MONOCRYSTALLINE RUTILE AND A METHOD FOR PREPARING THE SAME Application August 12, 1953, Serial No. $73,913

"i Claims. (Cl. 106-42) No Drawing.

The present invention relates to rutile single crystal boule modified with respect to certain physical properties, and to methods for the production of such boules.

Rutile is one of the three known crystal modifications of titanium dioxide. When substantially pure, a massive single crystal of rutile has gem-like properties with a very light straw color and reflectance, refraction and brilliance greater than that of a diamond. By means of the present invention asteriated rutile single crystals may be produced which have a substantially white color.

The principal object of the present invention is the preparation of asteriated rutile single crystals having a substantially white color. Another object of this invention is a method for producing an asteriated rutile single crystal boule. These and other objects of the present invention will become apparent from the following more complete description.

In its broadest aspects this invention contemplates an asteriated rutile single crystal containing a small amount of an oxidic compound of magnesium in solid solution and characterized by having a substantially White color and exhibiting asterism prepared according to the method of the present invention by progressively fusing a mixture of finely divided TiOz containing a small amount of an oxidic compound of magnesium in a flame carrying an excess of an oxidizing gas and thereafter subjecting the boule so formed to an oxidizing treatment.

In this description of the present invention the word boule is used in its currently accepted meaning to denote a characteristic shape or form of an artificially prepared massive single crystal having a rounded end, or meniscus, a more or less rod-like body portion and a tapering end, giving an overall appearance in profile of an almond, or carrot.

The general method to which the present invention relates is that described in another of assignees applications, that is in co-pending application Serial No. 286,853, filed May 9, 1952 and now abandoned, for the preparation of substantially pure rutile single crystal boules.

According to the preferred method of the present invention, there is employed as starting material a substantially pure TiOz which is very fine, fairly uniform and possesses antopen structure with units capable of being rapidly melted. A TiOz having an ultimate unit particle size of approximately 0.1 micron has proved especially satisfactory. In general, material having an average particle size about 5 microns should be avoided because such particles do not satisfactorily fuse under the conditions of the invention. A preferred method for preparing a TiOz starting material especially adapted for the production of rutile single crystal boules is described in U. S. Patent No. 2,521,392 wherein the double salt, ammoniumtitanium-sulphate is calcined until all of the ammonia and sulfate has been removed by volatilization.

According to the present invention a small amount of the oxidic compound of magnesium is mixed with the TiOz starting material in any convenient manner, for instance, by dry mixing. A more uniform mixture and 2,760,874 Patented Aug. 28, 1956 therefore a preferred mixture may be prepared by adding the magnesium compound as a powder, slurry, or solution, to the double salt ammonium-titanium-sulfate prior to calcination. The amount of magnesium compound to be added to the TiOz starting material to produce asterism is that which is suflicient to precipitate from the melt but not exceed the amount to prevent single crystal formation. This invention covers adding an amount of magnesium oxide, or any oxidic compound of magnesium which forms magnesium oxide at the temperatures employed, from about 0.2% to about 1.0% of the mixture by weight. Using amounts of magnesium oxide below about 0.2% does not produce boules which clearly show asterism while using amounts of magnesium oxide above about 1.0% tends to cause strains and fractures in the boule itself.

The color of the boule produced in the furnace is substantially black in color. Upon reoxidizing the boule a lighter color is obtained. It has been found that a substantially white color is obtained upon oxidizing the boule in an oxidizing atmosphere.

Although the black boule may be converted to a substantially white color upon heating in an oxidizing atmosphere at temperatures from 650 C. to 1000" C., in order for the boule to show asterism it is necessary to heat the boule up to a temperature of about 1100 C. to 1500 C. to precipitate the magnesium values in the single crystal boule. Upon such treatment a star rutile is obtained.

The rutile single crystal boule of this invention which contains titanium dioxide and magnesium oxide may, and often will, contain small amounts of various compatible substances which may enter as impurities in the raw materials. These substances may be only of a character and may be present only in amount so that the single crystal structure of the boule and its color are not substantially affected.

In progressively fusing the starting mixture of TiOz and a compound of magnesium oxide there should be employed a flame which carries an excess of an oxidizing gas, preferably oxygen. Preferably the fusion is carried out at a temperature of from 1830 C. to 1870 C. according to the method described in application Serial No. 286,853 supra, in which the particles of the starting mixture are entrained in a stream of oxygen in a tube constituting the center of a burner consisting of three concentric tubes. In this burner hydrogen is introduced to the flame through the intermediate tube and additional oxygen in excess over the amount required to combine with the hydrogen is introduced through the outer tube. Preferably, the ratio of oxygen to hydrogen by volume should be about 1:1 and should be not less than about 8.5 parts of oxygen to 9.5 parts hydrogen by volume.

The excess of oxygen in the flame is necessary because at elevated temperatures, titanium dioxide gives up oxygen and is converted to lower, or sub-oxides of titanium, e. g. Ti203. This loss of oxygen proceeds with considerable rapidity under conditions produced by an oxyhydrogen flame wherein the TiOz isfused and the loss is accelerated When the flame is neutral or reducing. Under such conditions the formation of lower, or sub-oxides of titanium is so great that mixtures of small crystals of various oxides of titanium are produced rather than a single crystal boule.

Even when operating with an excess of oxygen the boule as produced has a generally deep blue-black color indicative of some deficiency in oxygen. Therefore, in order to produce a boule of a lighter color, the blue-black has to be reoxidized.

As produced, rutile boule containing the oxidic compound of magnesium has a more or less frosted outer surface. When split, the interior surfaces of the pieces are vitreous and shiny. After heat treatment the boules may be cut and polished to form beautiful gems possessing a substantially white color and exhibiting asterism.

The boules may be prepared in the following manner. The feed material consisted of a mixture of titanium dioxide and 0.5% magnesium oxide. This feed material was progressively fused by a flame carrying an excess of an oxidizing gas. The fusions were carried out at temperature between 1830 C. and 1870 C. The boules were grown on a pedestal. The feed material was passed axially down through the oxygen-hydrogen flame onto a support which was aligned axially with such flame; the material melted in the flame being crystallized progressively by moving the support and flame apart axially of the flame. The blue black boules produced by the flame were then subjected to a subsequent oxidizing treatment at temperature from 1100 C. to 1500 C. to produce substantially white boules exhibiting asterism.

From the above description and by the examples presented, it has clearly been shown that asteriated rutile single crystal boules may be produced by adding small amounts of oxidic compound of magnesium.

While this invention has been described and illustrated by the examples shown, it is not intended to be strictly limited thereto and other modifications and variations may be employed within the scope of the following claims.

I claim:

1. An asteriated single crystal of synthetic rutile consisting essentially of titania and from about 0.2% to about 1.0% of an oxidic compound of magnesium.

2. An asteriated crystal of synthetic rutile consisting of titania and from about 0.2% to about 1.0% magnesium oxide.

3. A method for synthesizing an asteriated single crystal of rutile which consists of fusing a powder containing titania and from about 0.2% to about 1.0% of an oxidic compound of magnesium, crystallizing the fused material and heating the crystallized material in an oxidizing atmosphere of oxidizing gas until a substantially white crystal exhibiting asterism is obtained.

4. The method according to claim 3 in which the crystallized material is heated in an atmosphere of oxygen at a temperature between about 1100 C. and 1500 C.

5. A method for growing a single crystal of rutile which can be converted to a substantially white asteriated crystal by heat treatment in an oxidizing atmosphere, which method consists of periodically passing axially through a flame and melting therein a powdered composition containing titania and from about 0.2% to about 1.0% ofan oxidic compound of magnesium, accumulating and crystallizing the material so melted on a support aligned axially with such flame, and moving such flame and support apart axially of such flame.

No references cited. 

1. AN ASTERIATED SINGLE CRYSTAL OF SYNTHETIC UTILE CONSISTING ESSENTIALLY OF TITANIA AND FROM ABOUT 0.2% TO ABOUT 1.0% OF AN OXIDIC COMPOUND OF MAGNESIUM. 